This invention relates generally to devices for electrical connectors and, particularly, to isothermal connectors for thermocouple temperature measurement calibration systems.
Thermocouples have been used for many years to make accurate temperature measurements. A thermocouple consists of a junction of two dissimilar metals with wire leads connected to each half of the junction. The thermocouple generates a thermoelectric voltage that is a function of the junction temperature and the choice of the particular metals which form the junction. Because the thermoelectric voltage generated a function of temperature can be determined for particular thermocouple junction, a voltage-measuring instrument can be used to measure the junction voltage and convert the voltage measurement into corresponding temperature information through a simple voltage to temperature conversion algorithm. A variety of different metals for use in the thermocouple junction have been employed to cream different types of thermocouples to suit the requirements of different measurement tasks. Typical selection parameters for thermocouple types include the anticipated temperature range of the measurement application and the required measurement accuracy. "J-type" and "K-type" thermocouples are examples of commonly available thermocouples which have well known characteristics in the industry for temperature range and accuracy. Temperature measurement instruments that utilize thermocouple probes will commonly accept J-type and K-type thermocouples and provide proper conversion of the measured junction voltage into a temperature reading according to IEC 584 and NIST (National Institute of Standards and Technology) temperature/voltage tables.
The thermocouple may be connected directly to a measuring instrument, making the task of measuring the thermocouple junction voltage straightforward. More commonly, however, the thermocouple is connected indirectly to the measuring instrument via thermocouple extension wires and a thermocouple connector which plugs into a corresponding connector on the measuring instrument in order to accommodate the rapid connection of different thermocouple probes to the measuring instrument. A blade-type thermocouple connector with metal contacts of a specified type and dimension has been standardized in the industry.
Thermocouple connectors have an undesirable side effect. The metal-to-metal contact between the connector of the thermocouple and corresponding contact in the instrument connector forms another junction of dissimilar metals which contributes its own thermoelectric voltage to the measurement, thus making it necessary to take further steps to counteract the error voltage contributed to the measurement. One strategy is to measure the temperature of the connector junction in order to estimate and remove its thermoelectric voltage contribution from the measurement.
The temperature of the connector junction may be measured by a temperature sensor that is typically, but not necessarily, located near the connector. To ensure accurate temperature readings by the instrument, it is important to minimize any temperature differences between the two contacts of the connector which form two separate junctions, one for each wire that is connected to the thermocouple. Furthermore, it is important to minimize the temperature difference between the temperature sensor and the connector-to-connector junctions. A temperature difference between the two contacts of the connector, or between the temperature sensor and the contacts, results in measurement errors which degrade the overall accuracy of the temperature measurement and cause the temperature measuring instrument to make erroneous temperature readings. Therefore, it is desirable that a thermocouple connector be "isothermal" in that the temperature between the two contacts and between the contacts and the temperature sensor is maintained at substantially the same temperature.
U.S. Pat. No. 5,090,918 "Isothermal Termination Block Having A Multi-Layer Thermal Conductor" to Zoellick et. al, and assigned to Fluke Corporation, discloses an isothermal termination block that facilitates the connection of a plurality of thermocouple wire leads to a set of associated wire terminals. The isothermal termination block achieves improved thermal coupling between each pair of terminals and also between the terminals and a local temperature sensor mounted on the terminal block through the use of a multi-layer printed circuit board with embedded layers of thermal conducting material, allowing improved thermal conductivity in a substantially smaller physical size compared to prior art, among other advantages. However, this solution, while solving the problem of maintaining an isothermal relationship between the wire terminals and the local temperature sensor in a semi-permanent installation, does not address a concern regarding the settling time required for a stable measurement after the connection and re-connection of different thermocouple connectors.
Furthermore, instrument connectors are typically smaller in size than terminal blocks. Zoellick et at. does not address the problem of maintaining temperature stability in a small physical size. The particular problem of small physical size in instrument thermocouple connectors is illustrated in the common situation where a thermocouple connector of one temperature is inserted into an instrument connector of another temperature. Thermal mass is a measure of the amount of thermal energy an object is capable of storing. Settling time is the minimum amount of time required for the temperatures of the two metal-to-metal junctions that form between the two connectors to stabilize and equalize to a degree that is sufficient for the desired measurement accuracy. The problem is exacerbated by the fact that a blade-type thermocouple connector is polarized by making one pin larger than the other in order to prevent insertion of the thermocouple connector in reverse polarity into the instrument connector. Because the two pins are different sizes, the volume of metal between the pins necessarily differs and therefore the thermal mass of the pins differs, resulting in degraded measurement accuracy and longer settling times because the two junctions approach an equilibrium temperature at different rates. Prior art thermocouple connectors were not designed for isothermal operation nor optimized for settling time after connection.
Increasing the thermal mass of the electrical contact, allows the contact to more rapidly equalize its temperature with that of another contact of a different temperature of a lesser thermal mass when the two contacts are thermally coupled with each other. The greater the ratio of the thermal mass of one contact to the other, the lesser the amount of settling time required for the temperatures to equalize. Increased thermal mass of the contact also allows greater stability in the face of short-term variations in the ambient air temperature. Thus, it would therefore be desirable to provide a compact isothermal instrument connector with increased thermal mass that offers a reduced amount of settling time, increased measurement accuracy, and reduced susceptibility to changes in ambient temperature.